When you say RTF plane, do you mean the receiver, batteries and servos are included, or is the plane built without any radio gear?
If there is no radio gear at all then a decent 4-channel setup will cost around $140.
Try looking at the Futaba 4VF.
http://www.futabarc.com/radios/futj61.html
Or if you are after just the transmitter then you can buy these from some of the hobby stores and you would be looking at around $50.
Plane Insane has a good point though. Search the for sale forums and see what you come up with.
If you are buying a transmitter only, just make sure it is on the same channel as your receiver.
Hope this helps.